1. Field of the Invention
The instant invention relates to chromatography columns and methods of fabricating chromatography columns. More particularly, the instant invention relates to bed supports and restrictors for chromatography columns and to methods of fabricating such.
2. Technical Consideration and Prior Art
The trend in chromatography has been to move to higher pressures and smaller diameter columns for efficient solvent utilization and high column efficiency in high-performance liquid chromatography and for high column efficiency in supercritical fluid chromatography. By utilizing long chromatography columns or tubes, greater resolving power can be realized, allowing complex mixtures to be effectively separated.
Utilization of chromatography tubes having relatively small inner diameters in the range of preferrably 10-1000 .mu.m are utilized to accomplish state-of-the-art chromatography. By having small inner diameters, generally not greater than about 2000 .mu.m the volume defined by a chromatrography column is minimized. Accordingly, minute samples can be analyzed with state-of-the-art chromatrography columns. In order to increase the resolution of the sample components dissolved to a solvent within a chromatorgraphy column, the lengths of state-of-the-art columns are many orders of magnitude greater than the diameters thereof. For example, a chromatography column utilizing a liquid solvent might have a diameter of 250 .mu.m and a length of approximately 1 meter, whereas a chromatography column utilized with supercritical fluid chromatography might have a length of 19 meters and a diameter of approximately 80 .mu.m.
Generally, a chromatography column is packed with a sorbent which forms a chromatography bed. For example, the sorbent may be composed of microparticles such as C.sub.18 bonded phase particles having diameters in the range of 1-10 .mu.m which are suspended in a slurry and injected under pressure into the bore of the column. During injection of the slurry and during operation of the column, one end of the column must be fitted with a plug which may be for example a porous plug, in order to retain the sorbent therein. According to current practices, this is accomplished by inserting a glass wool plug in the downstream end of the column in order to form a support for the bed of sorbent. This approach to plugging the outlet end of the column has not provded totally satisfactory in that it is very difficult to insert the glass wool plug in such a small opening. Moreover, since there is nothing binding the glass wool plug to the wall of the column, the plugs on occasion are ejected from the columns due to pressure created within the columns during use thereof. This problem is addressed in U.S. Pat. No. 4,483,773 issued Nov. 20, 1984 to Yang, wherein alternatives to glass wool plugs are disclosed. These alternatives include utilizing narrow-bore tubing, wire, particles of a diameter larger than the constituent particles of the sorbent, and inserting a narrow-bore plug into a sleeve attached to the outside of the column. The patent to Yang clearly states that there is difficulty with the glass wool plug concept and resorts to a host of alternatives-the relative merits of which the instant inventors have little familiarity. Since the solvent in which the sample under analysis is dissolved must pass through the plug, the configuration of the plug directly affects the efficiency of the column. In part, the efficiency of the column is determined by the speed at which the solvent moves through the column and the amount of solvent which is utilized for a particular test. Generally, glass wool plugs provide high efficiency. Consequently, a desirability of any alternative to glass wool plugs should have an efficiency which at least approaches that of glass wool plugs.
In columns used for supercritical fluid chromatography (SFC) a restrictor is placed at the exit end of the chromatographic column. According to current practice, the restrictor is configured from a small-diameter capillary tube. Generally, the capillary tube forming the restrictor is butted or placed end-to-end against the chromatographic column and held in place by a union fitting or "butt-connector." Capillary tubing of Pyrex glass, fused silica and platinum iridium have been used as restrictors. The small-diameter capillary tube allows slow decompression of the mobile phase, or solvent, before detection. However, with this approach, relatively involatile solute molecules associate and condense along the walls of the small-diameter capillary tube. This results in both clogging of the opening and spiked peaks seen by the detector. In addition, laser-drilled orifices have been used as restrictors, but they are prone to blockage and breakage. Generally, these prior art restrictor designs are not entirely satisfactory in that they are separate from the columns and must be butted therewith or otherwise held in engagement therewith.
The prior art includes a number of teachings of porous plugs used with chromatography columns. For example, U.S. Pat. No. 3,440,864 discloses utilizing a porous stainless steel or fiberglass disc inserted into a chromatography column. U.S. Pat. No. 3,771,659 discloses a porous plug of any naturally porous or artificially porous material, such as Teflon, which is used with a chromatography column. U.S. Pat. No. 4,142,856 discloses a disc of microporous material formed from a polymer such as high-density polyethylene, which disc is inserted into a column. U.S. Pat. No. 4,181,853 utilizes a porous plug of sintered steel frit which is used in a chromatography system. U.S. Pat. No. 4,399,032 discloses a sintered metal terminator element having an outer rim which supposedly provides a seal with an outlet union retaining it. Each of these patents disclose arrangements of relative structural complexity wherein the plugs do not have the advantage of being unitary with the chromatographic column so that they effecitively seal with the walls of the column.
U.S. Pat. No. 2,723,756 discloses a cast-in-place plug made of plaster of Paris which is inserted in a large-diameter support cylinder. However, the plug is not actually in a chromatographic tube since the chromatographic column of this invention does not include a tube.